ISCSI: redo how we set up the events

ISCSI: redo how we set up the events

Call qemu_notify_event() after updating events.  Otherwise, If we add
an event for -is-writeable but the socket is already writeable there
may be a delay before the event callback is actually triggered.

Those delays would in particular hurt performance during BIOS boot and
when the GRUB bootloader reads the kernel and initrd.

But first call out to the socket write functions directly, and only set up
the write event if the socket is full.  This will happen very rarely and
this improves performance.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>